Determine if interstitial levels of adenosine are increased by ischemic excerise in humans, to test the hypothesis that endogenous adenosine contrubites to the sensory stimulation and reflex sympathetic activation evoked by ischemic excerise in humans and that endogenous adenosine acts as a local endogenous sympathetic modulator by blocking norepinephrine release in the forearm. We plan to study more subjects at this time.